Comparison of cation exchange capacity extraction methods for soil data harmonization and soil classification in Central and East Europe

Cation exchange capacity and base saturation are of crucial importance for soil characterisation and classification; however, the interchangeability of soil data for international assessment of soil productivity, soil classification, mapping, and modelling is limited due to the lacking comparisons between results obtained using different analytical methods in particular regions of the world. The aim of the present study was to analyse the relationships between cation exchange capacity and base saturation measured using the methods adopted in Central and East Europe, and 'standard' techniques required for soil classification suggested by World Reference Base (WRB). This study was carried out using 183 soil samples collected from Luvisols, Retisols, Planosols, Chernozems, Phaeozems, Cambisols, and Arenosols in Poland, representing a wide range of texture, pH, and organic carbon content. A close comparability was found in noncalcareous soils between the 'total sorption capacity' (T) measured in Central and East Europe as the sum of base cations and 'total' ('hydrolytic') acidity, with the 'standard' cation exchange capacity (CEC) measured using 1 M ammonium acetate buffered at pH 7. The close correlation between T and CEC values (in noncalcareous soils) facilitates reliable data recalculation and its application in global modelling, soil classification and mapping. For soils containing carbonates, CEC must be measured using reference methods, because no relationship exists between T and CEC. The 'potential' base saturation (V) derived on the basis of T may be reliably recalculated to 'standard' base saturation (BS), allowing a retrieval of archival data from Central and East European regional databases and published reports. Similarities between the values of cation exchange capacity and base saturation, whether measured or calculated using the local and standard methods, allow a positive verification of previously proposed correlations between the local soil taxa and the reference soil groups of the WRB classification and soil orders defined by USDA Soil Taxonomy. The pH values corresponding to 50 % of 'standard', 'potential', and 'effective' base saturation were estimated at 5.5, 5.2, and 4.8, respectively. Irrespective of the differences between current estimates and previously reported pH threshold values corresponding to 50 % base saturation, the obtained results confirm that field measurements of soil pH may be considered a substitute for laboratory-measured base saturation for some purposes, such as soil classification.